1. Field of the Invention
The present invention relates to an image forming apparatus. More specifically, the present invention relates to an image forming apparatus which utilizes a so-called charge injection type electrophotographic process in which processes such as charging, exposure, development, cleaning, etc. can be carried out almost simultaneously.
2. Description of the Prior Art
A conventional charge injection type of image forming apparatus is disclosed in, for example, Japanese Patent Application Laying-Open No. 58(1983)-153957. FIG. 1 is an illustrative view showing one example of this prior art image forming apparatus.
The prior art image forming apparatus 1 is constructed by arranging a developing unit 3 above the outer periphery of the photoreceptor 2 and a transfer unit 4 below the outer periphery of the photoreceptor 2 as well as arranging an LED array head 5 inside the photoreceptor 2. The photoreceptor 2 includes a cylindrical transparent substrate 2a made of a glass, and a transparent electrode 2b and a photoconductive layer 2c, which constitutes a photoconductive member, are laminated on an outer periphery of the substrate 2a. A voltage (V) of approximately 20 volts is applied as a developing bias voltage between the transparent electrode 2b and a magnetic roller 6 constituting the developing unit 3. A conductive magnetic toner 7 is absorbed onto a periphery of a developing sleeve 8 covering an outer periphery of the magnetic roller 6, whereby a so-called magnetic brush is formed. An ear end of the magnetic brush 9 is substantially brought into contact with an outer periphery of the photoconductive layer 2c. Therefore, an electric charge is injected into the photoconductive layer 2c from the developing bias voltage source through the conductive magnetic toner 7 so that the photoconductive layer 2c is charged to approximately the same potential as the developing bias voltage.
On the other hand, a light image projected from the LED array head 5 is incident on the photoconductive layer 2c from an inside of the cylindrical transparent substrate 2a to form an electrostatic latent image on the photoconductive layer 2c. At this time, the toner 7 is adhered on the surface of the photoconductive layer 2c from the magnetic brush 9, and therefore, a toner image is formed. The toner image is transferred onto a recording paper by the transfer unit 4. Then, remaining toners on the surface of photoreceptor 2 are removed by a cleaning force of the developing unit 3 and a magnetic force of the magnetic roller 6. Consequently, processes such as charging, exposure, development, cleaning and etc. are almost simultaneously carried-out by the developing unit 3 and the LED array head 5, and therefore, structure of an image forming apparatus as well as electrophotographic process can be significantly simplified.
In the above described method where the conductive magnetic toner is utilized in the charge injection type electrophotographic process, in a case of a direct transfer system in which the toner image is directly transferred onto the recording paper, it is required to use a high-resistance recording paper which is obtained by coating a specific material on a plain paper, and therefore, there is a problem that it is impossible to use a plain paper.
In addition, in a case of an indirect transfer system in which the toner image is transferred onto the recording paper via an intermediate member, although a plain paper can be used, the toner image formed on the photoreceptor must be transferred onto a plain paper via the intermediate transfer member such as a transfer belt, and therefore, components such as a cooling device for the transfer belt, a cleaning device for remaining toners on the transfer belt, a zigzag preventing device for the transfer belt, etc. are required. Consequently, there is a disadvantage that an image forming apparatus becomes large and a driving system thereof becomes complex.
Then, in, for example, Japanese Patent Publication No. 5(1993)-38950, there is disclosed a technique in which the toner image can be directly transferred onto a plain paper in the charge injection type electrophotographic process by utilizing a developing agent in which a conductive carrier and an insulative toner are mixed with a predetermined ratio.
In the prior art disclosed in Japanese Patent Publication No. 5-38950, since all charging amount necessary for developing is injected to the photoreceptor via the developing agent, a so-called background fog phenomenon occurs. More specifically, in this prior art device, since the insulative toner is first adhered to the photoreceptor, and resultingly, it becomes difficult for the photoreceptor to be charged, and therefore, there occurs a potential difference between the photoreceptor and the developing sleeve. Accordingly, the so-called background fog phenomenon occurs wherein the insulative toner is adhered at a surface of a non-image portion, i.e., a non-exposed portion.